Essentially all multi-megawatt scale photovoltaic (PV) power systems use a central power converter and building blocks of roughly 1 MW where DC power is collected in one location to feed a 1 MW DC-to-AC power converter or inverter. The power converter is connected locally to a distribution transformer to step up low inverter output grid-tie voltages to medium voltage distribution levels for final, system-level power collection. The advantage of this approach is inverter economies of scale. The disadvantages are that a single array ground fault or inverter failure will disable a megawatt of production, high energy DC arc potentials exist, maximum power point tracking accuracy is low compared to distributed power converter approaches, preventative maintenance is required, usable inverter lifetime is, at best, less than half that of the solar modules and inverter-specific site infrastructure costs are relatively high.
A second method, little used but a potentially emerging technology, is to use a number of low power PV string to DC power converters distributed throughout a 1 MW solar array field all sourcing power to a 1 MW DC-to-AC power converter and medium voltage distribution transformer. This solution provides higher DC collection voltages and therefore enhanced intrafield power collection efficiencies, provides greater PV maximum power tracking granularity and enables the DC-to-AC inverter stage to work at higher power conversion efficiencies. The disadvantages are that all central-inverter-related drawbacks are still in place; two-stage power conversion, PV-to-DC and then DC-to-AC, significantly limits system power conversion efficiencies, system complexity is high and the cost of fuses and disconnect switches rated above 600 Vdc (in most cases) and above 1000 Vdc (in all cases) negate any copper conductor savings.
A third method, proposed by micro-inverter manufacturers, involves using one PV to single-phase AC micro-inverter for every solar module and where one or two tiers of intrafield 60 Hz voltage step-up transformers would be required to facilitate AC power collection. This solution provides excellent system uptime because of the quasi-redundancy provided by a great number of low power inverters. Other benefits include DC arc hazard mitigation and the manufacturing potential for very high levels of power converter integration. The micro-inverter system drawbacks include inefficient, intrafield collection due to low AC inverter output voltages and/or lower tier 60 Hz step-up transformer losses, high system complexity, very low component-count-based Mean Time Before Failure (MTBF) numbers for the system, higher initial $/kW inverter costs and high inverter replacement maintenance costs. In addition, single-phase AC micro-inverters must use short-lifetime electrolytic energy storage capacitors or incur a cost premium for bulk film-type energy storage capacitors or suffer low power conversion efficiencies.
The present invention has all the advantages and none of the drawbacks associated with these three prior-art approaches.